How Does One Lose Weight?

I am 5'2 178lbs 37yrs old have very little appetite, but when I do eat I over eat but usually healthy foods. meaning I dont eat alot of junk food,fast food, or deep fried foods, not a couch potato, but currently not on a regular excersise routine my job consists of alot of regular lifting and moving. I cant seem to loose weight or keep it off when I do. my gain is mostly in my belly looking as if I am pregnant with twins. at my lowest weight I was 13 yrs old 4'11 and 98lbs in the last 15 years my lowest weight was 138lbs. and that weight loss was due to depression and stress because of divorce. what could be causing the lack of ability to loose the weight?